STOICHIOMETRY

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STOICHIOMETRY

• Calculations Based on
• Chemical Equations

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Iron (III) oxide reacts with carbon monoxide to
form iron and carbon dioxide.
3
2
3

• Fe2O3 CO ? Fe CO2

How many CO molecules are required to react with
25 particles of Fe2O3?
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Iron (III) Oxide reacts with carbon monoxide to
form iron and carbon dioxide.
3
2
3

• Fe2O3 CO ? Fe CO2

How many iron atoms can be produced by the
reaction of 2.5 x 105 particles of Fe2O3?
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STOICHIOMETRIC RELATIONSHIPS

• Formulas can also represent MOLES of substances
  involved in chemical reactions.
• Equations define reaction ratios, i.e. the molar
  ratios of reactants and products

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What mass of CO is required to react with 146
grams of iron (III) oxide?
Fe2O3 3CO ? 2Fe 3CO2
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What mass of iron (III) oxide is required to
produce 8.65 grams of carbon dioxide?
Fe2O3 3CO ? 2Fe 3CO2
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Limiting Reactants (Reagents)

• and Percent Yield

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Calculations need to be based on the limiting
reactant.

• Example 1 Suppose a box contains 87 bolts, 110
  washers and 99 nails. How many sets of 1 bolt, 2
  washers and 1 nail can you use to create? What
  is the limiting factor?
• 55 sets washers limit the amount

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Calculations need to be based on the limiting
reactant.

• Example 2 What is the maximum mass of sulfur
  dioxide that can be produced by the reaction of
  95.6 g carbon disulfide with 100. g oxygen?
• CS2 O2 ? CO2 SO2
• Start by balancing the equation

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Calculations need to be based on the limiting
reactant.

• Example 2 What is the maximum mass of sulfur
  dioxide that can be produced by the reaction of
  95.6 g carbon disulfide with 100. g oxygen?
• CS2 3O2 ? CO2 2SO2
• Now solve the problem

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• Example 2 What is the maximum mass of sulfur
  dioxide that can be produced by the reaction of
  95.6 g carbon disulfide with 100. g oxygen?
• CS2 3O2 ? CO2 2SO2

Which reactant is LIMITING?
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• Example 2 What is the mass of sulfur dioxide
  that can be produced by the reaction of 95.6 g
  carbon disulfide with 100. g oxygen?
• CS2 3O2 ? CO2 2SO2

O2 limits the amount of SO2 that can be produced.
CS2 is in excess.
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• Example 2 What is the mass of sulfur dioxide
  that can be produced by the reaction of 95.6 g
  carbon disulfide with 100. g oxygen?
• CS2 3O2 ? CO2 2SO2

134 g of SO2 can be produced in this reaction.
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Calculations need to be based on the limiting
reactant.

• Example 3 What mass of CO2 could be formed by
  the reaction of 8.0 g CH4 with 48 g O2?
• CH4 O2 ? CO2 H2O
• Start by balancing the equation

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Calculations need to be based on the limiting
reactant.

• Example 3 What mass of CO2 could be formed by
  the reaction of 8.0 g CH4 with 48 g O2?
• CH4 2O2 ? CO2 2H2O
• Now solve the problem

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• Example 3 What mass of CO2 could be formed by
  the reaction of 8.0 g CH4 with 48 g O2?
• CH4 2O2 ? CO2 2H2O

Which reactant is LIMITING?
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• Example 3 What mass of CO2 could be formed by
  the reaction of 8.0 g CH4 with 48 g O2?
• CH4 2O2 ? CO2 2H2O

CH4 limits the amount of CO2 that can be
produced. O2 is in excess.
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Many chemical reactions do not go to completion
(reactants are not completely converted to
products). Percent Yield indicates what
percentage of a desired product is obtained.
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• So far, the masses we have calculated from
  chemical equations were based on the assumption
  that each reaction occurred 100.
• The THEORETICAL YIELD is the yield calculated
  from the balance equation.
• The ACTUAL YIELD is the amount actually
  obtained in an experiment.

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• Look back at Example 2. We found that 134 g of
  SO2 could be formed from the reactants.
• In an experiment, you formed 130 g of SO2. What
  is your percent yield?

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Example A 10.0 g sample of ethanol, C2H5OH, was
boiled with excess acetic acid, CH3COOH, to
produce 14.8 g of ethyl acetate, CH3COOC2H5.
What percent yield of ethyl acetate is this?

• CH3COOH C2H5OH ? CH3COOC2H5 H2O

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Example A 10.0 g sample of ethanol, C2H5OH, was
boiled with excess acetic acid, CH3COOH, to
produce 14.8 g of ethyl acetate, CH3COOC2H5.
What percent yield of ethyl acetate is this?